1. Field of the Invention
During the process of drilling gas and oil wells circumstances sometimes occur which result in fire at the well head and in these instances it is necessary to close off the effluent from the well in order to extinguish the fire. Although various forms of devices have been previously known for the purpose of closing off a well casing string against the discharge of gas or oil from the upper end thereof, many of these previously known devices have not included structure effectively conducive to simple operation, quick operation and the ability to conform to the size of a particular drill stem in use.
2. Description of the Prior Art
Examples of previously known forms of well shutoff devices are disclosed in U.S. Pat. Nos. 799,967, 998,400, 1,560,859, 1,592,249, 1,692,302, 1,788,763, 1,909,304, 2,512,744, 3,102,709 and 3,554,480.